Genetic studies of the cycle gene in yeast along with protein analysis of its gene product iso-1-cytochrome c has led to the identification of UAA and UAG nonsense mutants, missense mutants, frameshift mutants and mutations of the AUG initiation codon. The nucleotide sequences of these mutational lesions were deduced from amino acid changes in altered but functional, iso-1-cytochromes c. In addition, enrichment tecniques have been developed for both forward and reverse mutations which are based, respectively, on the defectiveness and functioning of iso-1-cytochrome c for lactate utilzation. This iso-1-cytochrome c system and the defined cycl mutants are being employed in numerous investigations at the molecular level, including: (a) determination of the efficiencies of suppression and to identify the amino acid residues that are inserted by various nonsense suppressors; (b) determination of the specific action of numerous mutagens; (c) investigation of the mechanism of initiation of translation by using mutants with AUG codons at various sites and with nonsense mutations and frameshift mutations before and after these AUG codons; (d) determination of the relationship of recombination frequencies and nucleotide alterations; (e) determination of which specific amino acid residues are necessary or can be varied in functtoning iso-1-cytochrome c. Mutants of iso-2-cytochrome c are being isolated and the regulation of iso-1-cytochrome c and iso-2-cytochrome c is being investigated. BIBLIOGRAPHIC REFERENCES: Moore, C.W. and F. Sherman, Role of DN sequences in genetic recomination in the iso-1-cytochrome c gene of yeast. I. Discrepancies between physical distances and genetic distances determined by five mapping procedures. Genetics 79, 397-418 (1975). Liebman, S.W., J.W. Stewart and F. Sherman, Serine substitutions caused by an ochre suppressor in yeast. J. Mol. Biol 94, 595-610 (1975).